Diversification too — like all Value Investing — is counter intuitive. More diversification is normally assumed to be better, but it's not.
1984 Talk
Warren Buffett gave a talk in 1984 at Columbia Business School, known today as The Superinvestors of Graham-and-Doddsville.
In the talk, Buffett says:
"In this group of successful investors that I want to consider, there has been a common intellectual patriarch, Ben Graham."
"Walter [Schloss] has diversified enormously, owning well over 100 stocks currently."
"[Charlie Munger] set up a partnership quite the opposite of Walter’s. His portfolio was concentrated in very few securities and therefore his record was much more volatile but it was based on the same discount-from-value approach."
Warren Buffett, Columbia Business School: The Superinvestors of Graham-and-Doddsville (1984) [PDF].
1993 Letter
In his 1993 annual letter to shareholders, Warren Buffett writes:
"Portfolio concentration may well decrease risk if it raises, as it should, both the intensity with which an investor thinks about a business and the comfort-level he must feel with its economic characteristics before buying into it."
"If you are a know-something investor, able to understand business economics and to find five to ten sensibly-priced companies that possess important long-term competitive advantages, conventional diversification makes no sense for you. It is apt simply to hurt your results and increase your risk."
Warren Buffett, Berkshire Hathaway: Letter to Shareholders (1993).
But Buffett also writes in the same letter:
"Of course, some investment strategies - for instance, our efforts in arbitrage over the years - require wide diversification. If significant risk exists in a single transaction, overall risk should be reduced by making that purchase one of many mutually-independent commitments."
"Another situation requiring wide diversification occurs when an investor who does not understand the economics of specific businesses nevertheless believes it in his interest to be a long-term owner of American industry."
Warren Buffett, Berkshire Hathaway: Letter to Shareholders (1993).
The Intelligent Investor
Buffett also wrote the preface to Benjamin Graham's book — The Intelligent Investor — and called it "by far the best book about investing ever written."
"There should be adequate though not excessive diversification."
Benjamin Graham, Chapter 5: The Defensive Investor and Common Stocks, The Intelligent Investor
To Summarize
The passages above explain the principle of diversification perfectly. Concentration yields better results, but is best employed by those with exceptional skill and experience. Diversification protects the average investor from losses.
So there's really no "one size fits all" rule when it comes to diversification.
The idea is that it's not only more profitable — but also safer — to put your money in a few well researched holdings rather a lot of poorly researched ones. So the deciding factor here is if one has the time and opportunity to do the research well.
Peter Thiel
Peter Thiel, co-founder of PayPal and the first investor in Facebook, writes in his book Zero to One:
“As we said, even the best venture investors have a portfolio, but investors who understand the power law make as few investments as possible. The kind of portfolio thinking embraced by both folk wisdom and financial convention, by contrast, regards diversified betting as a source of strength. The more you dabble, the more you are supposed to have hedged against the uncertainty of the future. But life is not a portfolio: not for a startup founder, and not for any individual.”
Peter Thiel, Zero to One: Notes on Startups, or How to Build the Future.
Thiel's comments on the Margin of Safety in Venture Capital are related to this principle of focus as well.
Buffett Explains
Buffett gave the following detailed explanation of his views on diversification at the 1996 Berkshire Hathaway Annual Shareholders Meeting.
"We think diversification is — as practiced generally — makes very little sense for anyone that knows what they’re doing. Diversification is a protection against ignorance..."
"There is less risk in owning three easy-to-identify, wonderful businesses than there is in owning 50 well-known, big businesses."
Warren Buffett, Berkshire Hathaway: Annual Shareholders Meeting (1996).
Lynch on Diversification
In the following video, Lynch explains why the combination of Going Long and Diversification works.
"So you have flops. Maybe you're right 5 or 6 times out of 10. But if your winners go up 4- or 10- or 20-fold, it makes up for the ones where you lost 50%, 75%, or 100%."
Peter Lynch, Fidelity Investments: Lessons from an investing legend (2019).
Kiyosaki on Diversification
investor and real estate mogul Robert Kiyosaki too quotes Buffett to explain optimum diversification.